Greenpeace has been forced to close two of its regional offices and "considerably" reduce its staff in India because of a government crackdown on allegedly unlawful foreign funding of NGOs.

The organisation has been campaigning on environmental issues in India for nearly two decades but has clashed in recent years with Prime Minister Narendra Modi's government, which has accused the watchdog of violating the country's foreign funding laws.

Greenpeace India announced Saturday that the crackdown had forced the organisation to "shrink in size considerably" and close its offices in the capital New Delhi and the eastern city of Patna.

Nearly 40 employees -- one-third of its staff -- across India have been asked to leave, a former staff member told AFP on condition of anonymity.

Greenpeace India had its foreign funding blocked in 2015 as part of a nationwide crackdown on charities.

Since coming to power in 2014, the Modi government has cancelled the licenses of nearly 15,000 charities to receive money from abroad.

The main Greenpeace office in the southern city of Bangalore was raided by officials last year, and nearly a dozen of its bank accounts were frozen over alleged violation of rules.

The NGO has denied the allegations, and said it generates donations from within India.

"Greenpeace India is the collective voice of thousands of Indian donors, activists and volunteers," Diya Deb, campaign director at Greenpeace India, said in a statement announcing the cutbacks on Saturday.

"The government can only freeze our accounts and shut our offices but Greenpeace is an idea that can never be extinguished."

The move was "inevitable", the former employee said on Sunday, because Greenpeace India has been "struggling with finances after the government targeted it over funding".

Greenpeace India has been critical of government policies over their environmental impact, including coal mining and nuclear power. It has also campaigned over worsening air pollution across the country, which experts have blamed for nearly a million deaths.

Many charities critical of the Indian government have been in the crosshairs in recent years.

Last year, Amnesty International's office was raided by police over alleged funding violations. The watchdog described it as a move to "instil fear" among rights groups.